Titanium is used as a material for the construction of structures such as roofs and walls in seashore areas because of its excellent corrosion resistance against chlorides. Approximately 10 years have passed since the introduction of titanium for use in roofs and the like, and not a single case of corrosion has been reported.
Nevertheless, depending on the surrounding environment, titanium surfaces used for extended periods of time often exhibit discoloration to a dark gold color. This discoloration occurs due to the growth of an approximately 10 nm-thick titanium oxide layer on the titanium surface, with the discoloration producing a buffering effect which limits discoloration only to the outermost surface layer and therefore does not impair the anticorrosion effect of the titanium. However, it poses a problem from an aesthetic standpoint.
Since such discoloration is produced by growth of a titanium oxide layer on the titanium surface by acid rain which results in a buffering effect, removal of the titanium oxide layer can restore the surface to its condition before discoloration.
However, as titanium oxide is a chemically stable compound, it can only be chemically dissolved by coating the discolored sections with a mixed solution of nitric acid and hydrofluoric acid such as is ordinarily used for acid cleaning steps in titanium production processes. Restoration to the surface condition before discoloration is difficult, however, because the titanium base layer is also considerably dissolved. Mechanical methods also exist, and they include methods of polishing the discolored sections using polishing materials, but because the titanium oxide layer which is responsible for discoloration is only a few tens of nanometers thick, the titanium base layer surface also suffers polishing and surface outer appearance can thereby be altered. Prior art methods have accomplished removal of discolored layers by acid cleaning or polishing, with acceptance of a certain degree of outer appearance alteration of the base metal (for example, Materials and Process, 144th Fall Conference, CAMP-ISIJ Vol 15 (2002)-1306).
In addition, a titanium carbide layer in titanium surface which is another cause of discoloration is also present on the surface layer of the titanium base substrate under the titanium oxide layer discoloration. Therefore, as the titanium carbide remains on the titanium surface layer even after removal of the titanium oxide layer, rediscoloring upon prolonged use can still occur after cleaning.
Thus, removal of discolored sections of titanium and titanium alloy building materials remains a difficult task, while it is even more difficult to enhance discoloration resistance after removal.
In light of these circumstances, it is an object of the present invention to provide a cleaning agent for removing discolored sections produced on the surfaces of titanium and titanium alloy building materials without impairing the outer appearance of the base layers, as well as a discoloration removal cleaning agent and discoloration removal cleaning method which inhibit discoloration after the removal.